JP2007-10006 A discloses the hybrid-type construction machine in which a part of driven mechanism is motorized. According to the disclosed hybrid-type construction machine, an engine is directly coupled to a motor generator which functions as an electric motor and a generator, and a rotating shaft of the motor generator is connected to rotating shafts of first and second variable displacement pumps in series. For the first and second pumps, a third variable displacement pump which is driven by the motor generator is provided separately. The first pump mainly supplies fluid in a tank to a boom cylinder circuit for controlling a boom cylinder; the second pump mainly supplies fluid in the tank to an arm cylinder circuit for controlling an arm cylinder; and the third pump supplies fluid in the tank to a turning motor circuit for controlling a turning motor. Further, a discharge port of the third pump is connected to a pressure fluid replenishing circuit for replenishing pressure fluid to the boom cylinder circuit and the arm cylinder circuit.
In ordinary construction machines, as is the case with the construction machine disclosed in JP2007-10006 A, two pumps (first and second pumps) are coupled to the rotating shaft of the motor generator in series which is directly coupled to the engine. However, depending on operations of the construction machine, there are cases in which two pumps are not always necessary. In such cases, the fact that the two pumps are coupled to the engine leads to waste of energy. Further, in the case of the hybrid-type construction machine, frequency of use of one of two pumps tends to decrease. For example, if the turning motor is motorized, there may be a case in which one of two motors suffices for a combined operation of a boom-down operation and a turning operation.